OCR Chemistry A A-Level 4th June 2019 unofficial mark scheme

Hey guys dunno if I’m super wrong or not but a couple friends and I got 30-something mg? Have I done something majorly wrong or ****ed up the mole calcs 😂

Hey guys dunno if I’m super wrong or not but a couple friends and I got 30-something mg? Have I done something majorly wrong or ****ed up the mole calcs 😂

Whats people opinion on the grade boundaries for this paper?
I think they will stay the same so around 90 for an A*?

We don't need to know the colours VO2+ and V3+ for our spec. I wrote that before a lot of Fe is added, there is still a lot of VO2+ which is blue. Then as more Fe is added, Fe3+ is formed which is green. But then once all of the Fe is added, more of V2+ is formed causing it to turn violet? Idk if that's 100% correct tho??

I think this is is the answer you get if you work out the mass of iron - the question asked about the mass of the iron compound so it should have been ~301mg I think.

What did you put for the number of isomers on the multi choice ?

i got 3 im pretty sure. because there were the cis and trans isomers but also the optical one of cis. wasnt sure whether the the optical isomers counted as 2 though, so might've been 4

wasnt the question asking only for the number of stereoisomers not just isomers?

cis trans and optical are all stereoisomers

so was the answer 3?
damn i put 2

so was the answer 3?
damn i put 2

But it wasnt a graph of rate...thats why i cant understand

Any thoughts on upcoming paper 2...

was last years paper 2 difficult ?

Whats people opinion on the grade boundaries for this paper?
I think they will stay the same so around 90 for an A*?

1. Orbital question for carbon
D
2. Calculation of M
B (Calcium)
3. Which statement is false about G2 Metals
B
4. Properties of transition metals which are true
2 and 3
5. Ozone calculation
113.6? not sure
6. Stereoisomers of 4-ligand ion
3
7. Mass of atom given find the molecule of the gas
A (CH4)
8. Find N ox. no in Mg(NO2)2
B (+3)
9. Which is a redox reaction
D
10. Acid base pair
D (H3O+ and H2O)
11. Gradient of lnk against 1/T has value -4420
the value i got was 36.7 kj i think (C)
12. which of these had giant covalent, giant metallic and simple molecule
A. sulfur Aluminium and Silicon
13. Alkaline hydrogen Electrode potential true statements
?



16ai) Which block do Mg and Ba belong in and why [1 mark]
s block because their highest energy electrons are found in s subshell

aii Magnesium Abundance Calculation [2 marks]
25.9... (was meant to be 5sf)

bi) write equation for CaO reacting with water [1 mark]
CaO + h2O --> Ca(OH)2

iipH of BaO vs SrO [1 mark]
i put 12 and 13 probably will have leniency on this tbh

write down all the equations of state etc [3 marks]
2K(s) + 0.5O2(g)
2K(s) + O(g)
2K(g) + O(g)
2K+ + 2e- + O(g)
2K+ + O-(g)
2K+ + O2-(g)

B) find lattice enthalpy [2 marks]
cant remember my answer, something negative tho

ci) explain why ionisation energy of sodium is more than potassium's [2 marks]
K has larger radius, more shells more shielding so weaker electrostatic force of attraction between nucleus and highest energy electron so less energy needed to remove it

cii) explain why lattice enthalpy of sodium oxide is more endothermic than K2O [2 marks]
smaller ion size for Na hence stronger force of attraction to opposite ion (O2-) so more energy released when the bond forms

17a) Blood 6 marker, keeps blood at pH7.40 explain how it's able to act as a buffer and work out [HCO3-]:[H2CO3] pKa value was 6.38 [6 marks]
H2CO3 -> HCO3- + H+
adding alkali/OH- means alkali reacts with the H2CO3 and equil. shifts to the right(towards HCO3-)
Adding acid/H+ means H+ reacts with the HCO3- and equil shifts to the left towards H2CO3
Acts as buffer because acid in excess
ratio was 1:10.5

17b) Explain in terms of ligand substitution how Haemoglobin n o2 work etc and how CO is toxic [3 marks]
o2 binds to Fe2+ ion, gets carried round body, CO2 subs it, CO2 released at lungs and expired
CO is toxic because it binds more strongly that O2 and binding is irreversible so doesnt let O2 bind any more
being constantl exposed to CO results in death because of O2 deprivation

18a) Find S and show its feasible explaining with a reason [4 marks]
Got smth like -1110 i think, feasible because it's less than 0 (or is negative) hence entropically feasible

18b) Find H2S formation enthalpy (clapped) [3 marks]
should be like -20.9 or smth

19a) Show expression with units [2 marks]
KP= p(SO3)^2 / {p(SO2)^2 x p(O2)} units atm-1

19b) Find kp value [5 marks]
got 27.2

19c) she repeats the experiment and kp increases, explain if temperature she repeated it at was lower or higher [2 marks]
reaction originally was exo thermic in forward direction so if eq shifted to the right then the temperature must have been lower(le chatelier)

d) explain with a diagram boltzmann dist. curve to show catalyst effect on rate [3 marks]
curve drawn labelled yaxis(no of molecules) x axis (energy) Ea and Ec labelled Ec is lower than Ea so more molecules with required activation energy hence more freq/successful collisions so faster rate

20ai) grey/green Precipitate forms what is it [1 mark]
Cr(OH)3

aii) Cr(OH)6- draw 3d structure (not sure if it asked for bond angle) [3 marks]

bi) yellow colour in step2 what was the ion(h202 was added) [1 mark]
(Cro4)2-??? not sure


bii) What colour is the mixture when (cr2o7) is present [1 mark]
Orange

ci) write e- config of v3+ [1 mark]
1s2 2s2 2p6 3s2 3p6 3d3

ii) using E potentials from box explain the colour changes blue(original vo2+) to green to violet [3 marks]
Fe was added in excess so using systems 2 and 4, Fe has more negative potential so would be oxidised to Fe2+ and Fe2+ causes green, hence VO2+reduced. Then using 2 and 3 ( cant remember the systems now) another oxidation occurs and a purple colour forms

iii) write equation for first change (blue to green)[1 mark]
Fe + VO2+ --> idk wht i wrote here smth like Fe2+ and V3+ idk

ACid questions
22ai) calculate acid of c2h5cooh with conc of 0.08moldm-3 [2 marks]
2.98

aii) show that only 20cm3 of 45cm3 0.01M NaOH is only needed to neutralise the acid [1 mark]
just use n=cv and use stoichiometric ratios

aiii) find the final pH of the buffer [4 marks]
12.55

bi) draw the pH curve for this reaction [3 marks]
strong base weak acid curve starting at 2.98 on the left and going to the top right at 12.55 like an S shape
vertical straight lines at x= 20cm3

bii) Explain which indicator [2 marks]
bromophenol, explain that the range of the vertical line lies is similar to range of indicator

c) student repeats it with HCN instead of C2H5COOH state one difference and one similarity between the graph you drew and the graph for HCN [2 marks]
similarity: same equivalence point/vertical line in same place
difference: Ka for HCN is smaller so the range of the graph would be bigger (i think)

d) student predicted HIO3 pH but measured pH was higher, explain why [2 marks]
He's assumed that the equil. pH is the same as acid pH but it partially dissociates hence pH would be greater as H+ concentration would increase


Final Rate equation question [6 Marks]:
From the graph of student 1, the concentration of Br2 decreases at a constant rate. Therefore it has to be zero order w.r.t Br2.
From the graph of student 2, the rate increases linearly with the reactant. Therefore, rate is directly proportional to the concentration. So the reaction is 1st order, w.r.t this reactant.
From the calculations, the reaction is 1st order w.r.t the H+ ion concentration.
rate constant was 3 point smth to the powr of 10 to the minus 4 i think